Off Road Truck Axleshaft Facts

Nobody likes to get the shaft in the proverbial sense. But getting the right shaft for your axle can keep you from being stuck in the yuck with no torque to the tires. This article hones in on the materials and manufacturing of OE and aftermarket axleshafts. We’re limiting it to rear shafts; front shafts might be covered in an upcoming issue.

Axleshafts begin as raw bars, 1541H in this case. Foote Axle & Forge’s oven heats the bars almost to the melting point, a process known as hot forging.

Most readers know that steel is a generic name for various iron alloys, or mixture. Iron (Fe) is steel’s base metal, with carbon (C) and such other elements as manganese (Mn), phosphorous (P), sulfur (S), silicon (Si), nickel (Ni), chromium (Cr), molybdenum (Mo) and vanadium (V) sometimes added to affect hardness, machinability, and heat-treatability.

Axleshafts typically use medium-carbon steel. Carbon increases strength but makes the steel less ductile or flexible. Aftermarket alloy shafts often have a higher carbon content than OE shafts and also introduce other metals such as chromium, molybdenum, and nickel into the mix. These metals increase strength and improve case hardening.

Upset forging forms the axle flanges (yellow-hot in the image). The heated bar is squished into a die under high pressure, kind of like flaring a brake line but on a much larger, hotter scale.

Here’s an overview of the AISI (Ameri-can Iron & Steel Institute) material grades most often used for automotive axleshafts. (The Society of Automotive Engineers tends to use AISI’s designations; MIL-Spec, ASTM, ASM, and international designations can vary.) The raw stock sometimes includes abbreviations that refer to the manufacturing process: H is hot-rolled, CD is cold-drawn, A is annealed, and Q is quenched, for example.

The first two numbers refer to the alloy and the last two the average carbon content. The 1xxx numbers are primarily the Carbon Group, the 4xxx has many Nickel-Chromium-Molybdenum Group (chromoly) members, and the three-number designations are considered aircraft-grade steels.

1340: This high-manganese grade was the OE material years ago. Many early Dana/Spicer axleshafts used 1340. Modern higher-performance applications need stronger material.

Heat treating/case hardening is the most critical step. After the shaft is cut to length and splined, Foote’s CNC induction-hardening machine precisely controls the depth and hardness of the treatment. The steel is heated to between 600 and 700 degrees F then quickly cooled. Each shaft is hardened individually.

1040: OE axleshafts are typically made from induction-hardened 1040 because it strikes a compromise between strength and ductility. 1040 is also easier to machine than harder alloys.

1050: Thanks to its higher carbon content, 1050 is about 38 percent stronger than 1040. It is used in certain OE applications and also for some aftermarket OE-replacement shafts.

1541: This high-alloy grade is popular with aftermarket manufacturers.

1541H: An even better aftermarket shaft material, this grade adds silicon to 1541 to increase strength and heat-treatability. 1541H can be 50 percent stronger than OE 1040 and about 12 percent stouter than 1050.

4140: 41xx designates the chromoly group. Chromium offers three benefits: improved hardness, better elasticity during quenching, and greater corrosion resistance. Molybdenum and nickel further increase hardness. This steel is also commonly used for U-joints, spindles, and camshafts.

4340: Also in the chromoly family, 4340 is about twice as strong as OE 1040. It is also ductile enough to absorb the shock of abrupt acceleration, taking some load off of the differential. 4340 is popular for performance-aftermarket front shafts because it is strong and more affordable than some of the more exotic alloys.

300M: Also known as 4340M, it is similar to 4340, only with vanadium added plus additional silicon and slightly more carbon and manganese. It is mainly used in aircraft applications where high strength and ductility are required for such components as landing gears. 300M is also normally through-hardened and is about 150 percent stronger than OE 1040. It is expensive, harder to machine than other shaft materials, and manufactured in much lower quantities than the other steel grades.

After the shaft is heat treated, Foote uses dial indicators to check for a consistent centerline. Any run-out is corrected.

Hy-Tuf: (ASM-6418, SAE 4625M4, MIL S-71083, aka Maxi-Drive) is a chromoly well known in drag racing circles. Strange Engineering and others sell Hy-Tuf shafts that have a Rockwell (HRC) hardness rating of 46-48 throughout. (Hy-Tuf is generally through-hardened.) The recipe is high in silicon and manganese. Hy-Tuf is more affordable and available than 300M.

8620: Also in the Nickel-Chromium-Molybdenum Group, this low-alloy (lower carbon) grade isn’t viable for drive axles—tensile strength is about half of the 1040 material typically used for OE shafts. 8620 is commonly used for components that need a hard surface to combat wear and a ductile core, such as ring-and-pinion gears.

The shafts then go to a grind station. Finish tolerances are ground per customer specifications.

Each steel grade has an acceptable range of carbon and other elements. (Strength ratings in the accompanying chart are approximate; a shaft’s actual strength is determined by the specific recipe used by the steel mill, by the hardening process, and by size.) The cost of the finished product often reflects the quality of the materials. Raw steel that is consistently on the upper end of spec tends to cost more than shipments that vary from load to load or are on the lower end of the acceptable range. Harder alloys are more difficult to machine, which also adds to the cost.

Manufacturing
Here is an overview of rear axleshaft manufacturing.

Splines:• Hobbed: This process carves involute (curved) splines into the shaft. Differential side gear splines are involute, so torque is spread more evenly when the splines align. Some smaller-volume businesses straight-cut their splines. This concentrates the stress on only part of the tooth instead of spreading the load more evenly. Straight cutting can also penetrate below the shaft’s case-hardening.

Hobbed splines (top) are cut into the shaft. Rolled splines (bottom) don’t remove material, so they are stronger and look better. Note the longer lead-in/ramp-up area on the hobbed splines’ inboard side.

• Rolled: A machine with expensive dies forms involute splines under high pressure. No material is removed.

Major Spline Diameter: The maximum distance across the splines (as determined by a caliper).

Minor Spline Diameter: The diameter at the base of the splines, which may or may not be the shaft’s minimum diameter.

Minimum Diameter: The skinniest part of the shaft, which is sometimes the minor spline diameter but can also be a necked-down area somewhere else on the shaft.

Diametrical Pitch: A mathematical calculation devised to keep the splines’ centerlines constant at any diameter. The current standard is 24-pitch. This means that the midpoints of the splines on a 24-spline shaft are 1 inch in diameters, the midpoints for a 35-spline shaft are 11⁄2 inches in diameter, and so on.

Pressure Angle: A spline’s tooth angle. It varies among manufacturers and is not interchangeable: For example, 30-degree Dana inner splines aren’t compatible with 45-degree Ford or Toyota differentials, even if the spline count is the same. Wider-angle splines produce slightly stronger shafts because the minor spline diameter is greater.

Generally, more splines means a larger-diameter, stronger shaft (the 14-bolt 30-spline is an exception). Just as fine-thread fasteners spread the load over a greater surface compared to coarse threads, smaller teeth/more splines are stronger than “coarse” splines, which are cut deeper into the shaft (minimizing the minor diameter). According to Dynatrac, performance aftermarket shafts for big-tired 4x4s start at 35-spline.

Hardening/Finishing Treatments
Hardening treatments increase shaft strength by altering the metal’s crystal-line structure. Heat creates carbon atoms, which are trapped thanks to prompt quenching with oil or water. The Rockwell scale, expressed in HRC, is the most common hardness measuring system for axleshafts.

This cross-section shows induction hardening. The dark outer area’s hardness has a rating of about 50 Rockwell (50 HRC), compared to the lighter inner portion at 18-20 HRC. Street and off-road shafts need some flex for cornering and suspension travel, so their centers need to have greater ductility compared to through-hardened shafts intended for drag racing.

Through Hardening: The entire piece of metal is theoretically hardened uniformly from case to core.

Induction Hardening: The method most commonly used for axleshafts. An electromagnetic field, with a current that can be varied depending on the material’s diameter and the desired depth of the hardened layer, heats the metal. Quickly thereafter the shaft is cooled.

Nitriding: This process uses a high-nitrogen solution such as ammonia to harden the surface while the metal is heated in an enclosure.

Other metal treatments and finishing processes further increase a shaft’s durability. Some of the possibilities are as follows.

Polishing: Micropolishing increases strength by minimizing surface irregularities and stress risers. 4340 chromoly has a fairly tight grain and does not benefit as much from polishing as the carbon-group steels do. Bearing/seal areas are normally ground or polished to minimize scoring or maximize sealing of these contact areas.

Cryogenic Freezing: Nitrogen gas is used to cool the part to about 300 degrees F below zero. This tightens the metal’s grain and improves fatigue-resistance more than overall strength. It is one of the more expensive treatments.

Black Oxide: This coating is primarily for resisting corrosion.

Gun Drilling: This process bores out the shaft’s core. Done properly, it saves weight with minimal strength loss. Gun drilling and cryogenic freezing aren’t in high demand in the value-conscious recreational 4x4 market. These options are investigated mainly by racers looking for every available edge.

The photos and captions here show how Foote Axle & Forge builds shafts from raw round bars. Many well known axle “manufacturers” are actually finishers, buying blanks from companies such as Foote and then finish-machining them as necessary for desired length, spline spec, and wheel bolt pattern. Finishers sometimes offer optional services like gun drilling and cryo freezing.

Design
In addition to metal grade and thickness, an axle’s profile helps determine its overall strength. Here is an overview of the popular rearend styles.

The three prominent rearend axleshaft wheel-end configurations for light-duty trucks are tapered (top), flanged (middle), and full-floating (bottom), with flanged being the most common.

Tapered Two-Piece: This style has a separate shaft and flange. Semifloaters use a key in a keyway to mate their shafts and flanges. Nuts secure the outboard ends of the shafts to the flanges in this style.

Semifloating: This style commonly uses a one-piece flanged axle to connect the differential and wheel. Semifloating axleshafts do double-duty: They transfer torque and support the vehicle’s weight. Some semifloating axles use C-clips inside the differential to hold the shafts on the vehicle, rather than a pressed-on bearing and flange. Because the shafts’ flanges bolt to the wheels and not to the axlehousing, shaft breakage can allow the tire/wheel to move away from the axlehousing. C-clip eliminator kits for popular axles such as the Dana 35C, Ford 8.8, and Chevy 12-bolt include axlehousing ends, pressed-on bearings, and bearing retainer plates to positively retain the axleshafts to the axlehousing. DIY eliminator kits that have bolt-on housing ends tend to allow more side-loading on the bearings and shafts than weld-on ends. However, the welded ends require an alignment bar for installation to ensure that the housing tube’s centerline stays true.

Full-Floating: This rearend style is preferred for vehicles that carry heavier loads, typically 3⁄4-ton and up. As with front ones, full-floating rear axleshafts connect to hubs, which turn the wheels. The hub supports the vehicle’s weight. Should a full-floating shaft break, the hub and wheel normally remain bolted to the axlehousing. Full-floating shafts can either have splined outboard ends that key into the hubs or drive flanges that bolt to the wheel hubs.

Full-floating shafts come in two configurations. Frontend-style outboard splines (bottom) slide into a splined hub. Typical 3⁄4- and 1-ton applications use flanged full-floaters that bolt to the wheel hub.

Strength
The accompanying chart (right) gives thumbnail yield strength (the point before the metal is permanently deformed) for axleshafts of various diameters. That information can be used to calculate an axleshaft’s torque capacity using the formulas above. Radius and diameter are in inches; to convert torque capacity to pound-feet, divide the pound-inches by 12.

So, how much torque capacity does your 4x4’s axleshafts need? Maximum drivetrain output torque can be calculated by multiplying engine crankshaft torque by crawl ratio, then using a 0.85 correction factor to compensate for driveline slop. Here is the calculation for a stock ’12 JK Rubicon with a five-speed automatic.

250 lb-ft engine x 3.59 First

x 4.0 T-case x 4.10 axle = 14,719

14,719 x 0.85 parasitic loss = 12,511

12,511 = 3,128 lb-ft per shaft
4 axleshafts

Rear shafts are often quasi-equal in length; they theoretically split the load in half under normal tractive situations and when a locker or spool is engaged.

Shaft Selection
So what shaft is right for you? The facts and figures here will hopefully help narrow down the options. Internet forums are another possible resource, if you can find knowledgeable posters who are running your proposed setup and who attack similar terrain. Relevant posts often recommend certain manufacturers. Using these manufacturers’ tech lines can zero in on what is best now and possibly down the road, based on current and intended vehicle setups, budgets, and so on.

The Randy’s Ring & Pinion Differentials book, by Jim Allen and Randy Lyman, is an excellent reference for all things axle-related. It covers both OE configurations and aftermarket upgrades.

The bottom line on axleshafts: As with many things, price is usually proportional to quality. Higher-end shafts often involve more manufacturing/quality-control steps and use better raw materials. Many axle/drivetrain specialists offer both budget-conscious shafts (which some even label “import”) as well as premium domestic ones.

Luckily, even budget aftermarket shafts are usually significantly stronger than OE ones. Many are also backed by confidence-inspiring warranties (which, unfortunately, won’t help solve trail failures since cost of labor—which can be significant depending on axle style and where you are when it breaks—is normally excluded under the warranty). Getting the right shaft for the job will help keep you from getting shafted on the trail.